Efficacy of Epid-Based In Vivo Dosimetry and Calibrated CBCT Images for a Timely Lung Cancer Replanning

Abstract

The tumor volume shrinkage during the treatment of lung cancer can require an adaptive plan. The aim of this study is to investigate the efficacy of electronic portal imaging device (Epid) in vivo dosimetry (IVD) combined with calibrated cone beam computed tomography (CBCT) scans for a timely replanning. Sixteen lung cancer patients treated with volumetric modulated arc therapy were enrolled for this study. They underwent CBCT and Epid-based IVD verification twice a week and a second scan (CT2) in the middle of the treatment for a morphological reevaluation. The accuracy and reproducibility of the treatment delivered was assessed with: i) IVD tests by a quality control program that supplies three indices: the ratio R between the reconstructed and planned isocenter doses and g%, gmean (gamma analysis of the electronic portal images). They were considered in tolerance if R within 5%, g % >90% and if gmean <0.4; ii) a recalculation of the reference treatment planning (TPref) using CBCT scans acquired along the treatment and calibrated by the same quality control software. Therefore, the agreement of the DVH dosimetric indices D95%(PTV), V20Gy (lung), D1cc (spinal cord), Dmean(heart) with the ones computed on the planning CT(CTplan) was verified; a DVH indices difference of 3% was considered acceptable. IVD tests resulted out of tolerance for three patients before the scheduled CT2; DVH indices obtained by the plan recalculation on calibrated CBCT confirmed this discrepancy. For these patients, TPa was carried out on CT2 and compared with TPa computed on CBCT scans. In the table below, the results obtained for one of these three patients treated with 60Gy in 20 fractions over four weeks for a lung metastasis were reported; IVD tests and DVH indices out of the tolerance level were highlighted in bold. The warnings supplied by Epid-based IVD can offer an early advice for a replanning step. The adaptive plan computed on calibrated CBCT can be timely performed and can provide DVH indices with an accuracy of 3%.Abstract TU_29_3183; Table 1IVD Test ResultsDose CBCT vs CT (DVH indices difference)weekCT scan/ Treatment PlanRg%gmeanPTV (D95%)Homolateral Lung (V20Gy)Spinal Cord (D1cc)Heart (Dmean)week 1CTplan/TPref0.9795%0.171.1-1.71.21.9week 1CTplan/TPref0.96100%0.020.8-2.51.21.9week 2CTplan/TPref0.9998%0.093.12.31.21.9week 2CTplan/TPref0.9481%0.417.06.6-0.92.3week 3CTplan/TPref0.9386%0.358.05.90.60.9week 3CT2/TPa1.0098%0.082.12.11.0-2.2week 4CT2/TPa0.9996%0.091.00.90.0-2.8week 4CT2/TPa0.9996%0.050.81.8-1.1-2.0 Open table in a new tab